Intrinsically Optimizing Charge Transfer via Tuning Charge/Discharge Mode for Lithium-Oxygen Batteries
Lithium-oxygen batteries have an ultrahigh theoretical energy density, almost ten times higher than lithium-ion batteries. The poor conductivity of the discharge product Li 2 O 2 , however, severely raises the charge overpotential and pulls down the cyclability. Here, a simple and effective strategy is presented for regular formation of lithium vacancies in the discharge product via tuning charge/discharge mode, and their effects on the charge transfer behavior. The effects of the discharge current density on the lithium vacancies, ionic conductivity, and electronic conductivity of the discharge product Li 2 O 2 are systematically investigated via electron spin resonance, spin-alignment echo nuclear magnetic resonance, and tungsten nanomanipulators, respectively. The study by density functional theory indicates that the lithium vacancies in Li 2 O 2 generated during the discharge process are highly dependent on the current density. High current can induce a high vacancy density, which enhances the electronic conductivity and reduces the overpotential. Meanwhile, with increasing discharge current, the morphology of the Li 2 O 2 changes from microtoroids to thin nanoplatelets, effectively shortening the charge transfer distance and improving the cycling performance. The Li 2 O 2 grown in fast discharge mode is more easily decomposed in the following charging process. The lithium-oxygen battery cycling in fast-discharge/slow-charge mode exhibits low overpotential and long cycle life.